Electric wave filter



propagation.

the construction of "such filters.

Patented Nov. 1, 1927.

UNITED STATES PATENT OFFICE.

KENNETH S. JOHNSON, OE JERSEY CITY, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TBIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., -A CORPORATION 01' NEW YORK.

ELECTRIC WAVE FILTER.

Original application area This invention, relates. to electric wave filters adapted to be employed in a line such as a telephone line 'to permit the passage of currents of certain frequencies and to prevent the passage of currents of other frequencies an has for an object to improve This is a division of my previous application Serial No. 434,388, filed December 31, 1920, patented Janua 11,1927,No. 1,613,952. 1

A further 01)]6013 of this invention is to produce filters of'various types which have the characteristics of types'ot filters in general already known, but which may be manufactured from a less amount of material thus reducing the cost and size of the filter.

In accordance with the invention, filter sections are employed which have similar impedances spaced apart in the direction of currentpropagation but having mutual impedance therebetween and more impedances located between the mutually related impedances. The effect of mutual impedance in general is to add impedance to the common portion without the use of additional apparatus. v

This invention is' particularly related to the species of the invention in which the impedances having mutual impedance therebetweenare included in arms of the filter which are in shunt to the line of current The invention will be more clearly understood from the followingdetailed description taken in connection with the accompanying drawing in which Fig. 1 represents schematically a filter composed of a plurality of similar filter sections embodying the invention, and

Figs. 2 and 3 show filters without mutual inductance which are electrically equivalent to the filter of Fig. 1.

which have one or December 31, 1920, Serial No. 434,388, Patent No.'1,613,952. Divided and this application filed August 1925. Serial No. 53,077.

Fig. 1 shows a filter 01' three sections, each of which comprises a series condenser 0 and two shunt inductances L wound in parallel aiding relation on a magnetic core 4. By parallel aiding relation is meant that relation in which the coils are connected in parallel in such a direction that the flux produced by the current in one coil is added or increased by the current flowing in the other coil. 'The result of the mutual inductance between these two coils is a transformer action, as is well known, such a transformer can be represented by an equiv-- alent T or II network. Thus, Fig. 2 shows a filter, each section of which is formed by replacing the two mutual inductive coils L by their equivalent T network in accordance with the well known principles relating to transformers. Similarly, Fig. 3 is formed by the use of the equivalent II network.

In Fig. 2, the transformer is replaced by two series inductances LM and a shunt inductance M where L represents the inductance of the coils and M the value of the mutual inductance therebetween. In Fig; 3, the transformer is replaced by a series inductance and two shunt inductance! LI MI W a high pass filter section of the form shown. in Fig. 8 of U. S. patent to G. A. Campbell 1,493,600 dated May 13, 1924. As is pointed out in that patent and in G. A. Cam bell Patent 1,227,113, dated May 22, 1917, a l ter having a plurality of sections, each having Examination of Fig. 3 shows that it is i a total series impedance Z, and a total shunt impedance Z will transmit a band of frequencies whose limits are determined by the equations:

1 2; and Z m i. v a, t in the filter of Fig. 3,

1 id l Assuming 1 2 Assuming =0, we get w==a orf=a 9 1 f 2n,/ 2o L M) Therefore, the filter of Fig. 3 and hence that of Fig. 1 also will transmit freely all frequencies from l 211 /2065 M a It was shown in Patent No. 1,493,600 above referred to, that the attenuation of the filter section is infinite when:

1 1 (L 0 f 211 /L C Substituting in this equation If this frequency of infinite attenuation is designated as for and the cut-01f frequency be f, and the ratio be a, it will be seen that aea'aesc a and o=2l'lf, f being frequency.

be free from hysteresis and eddy-current losses or an air core is used, there 1 be very little resistance efiectively in the series arm as compared witha filter in which a physical inductance is used in the series arm since in the latter case the direct current resistance of the winding cannot be eliminated. It therefore follows that the maximum. attenuation of the filter can be made to more nearly approach infinity by the use of mutual inductance.

This invention is not limited to the particular filter section shown but may be greatly modified, without departing from the spirit of the invention as defined in the a ppended claims. For example,-other circuits such as series or parallel resonant. circuits may be substituted for the condenser C or may be connected in shunt to the inductances L to give to btherfilter characteristics.

What is claimed is:

1. An electric network adapted to be con-=- nected between two sections of a circuit, said network comprising a condenser connected in series between said sections, and an inductance at each side of said condenser, said inductances being in shunt to said circuits and being so related as to have mutual inductance.

2. A wave filter section comprising a plurality of circuit branches disposed in series and shunt relationship with respect to the direction of wave propagation, reactive impedances associated in said branches, said impedances including a pair of coupled inductances included intwo of said shunt branches which are separated by a series branch, said inductances'efiectivelyadding an inductance element to said series branch,

the impedances andsaid element cooperating to permit the filter to pass freely a band of frequencies while attenuating other frequencies.

v 3. A wave filter section accordingto claim branch and a pair of inductively coupled 2 in which said pair of con led inductances coils included in two of said shunt branches are connected in parallel ai 'ng relation. which are separated by said series branch.

4. A wave filter section comprising a plu- In witness whereof I hereunto subscribe I rality of circuit branches arranged in series my name this 26th day of August, A. D.

and shunt relationship to the direction of 1925. wave propagation, a condenser in a series. KENNETH S. JOHNSON. 

